Dual connectivity for Long Term Evolution (LTE) provided by a macro cell and a smaller cell has been specified in standard document 3GPP TS 36.300 V12.5.0 (inter alia in Sects. 4.9, 6.5, 7.6 and 10.1.2.8). In LTE dual connectivity, a user equipment (UE) connects concurrently to two radio access nodes (also referred to as evolved Node B or eNB in LTE), which are providing radio access in macro cell and the smaller cell, respectively. One eNB is an anchor point for control signaling, which is referred to as a Master eNB (MeNB). That is, the MeNB is connected to a Mobility Management Entity (MME).
The other eNB is referred to as a Secondary eNB (SeNB). The SeNB increases user data throughput in addition to the radio communication with the MeNB. To this end, the MeNB and the SeNB operate on mutually exclusive radio resource elements. The coordination between the MeNB and SeNB includes transmitting different packets from the respective eNB to the UE to increase the throughput received at the UE. White Paper “LTE Small Cell Enhancement by Dual Connectivity” by the Wireless World Research Forum discusses small cell enhancement by dual connectivity.
Next-generation Radio Access Technologies (RATs) use frequency bands at higher carrier frequencies, e.g., in the range of 2.6 GHz to 15 GHz or up to 100 GHz. High-gain beamforming allows compensating negative effects due to unfavorable radio propagation properties at the higher carrier frequencies. However, a serving beam of the high-gain beamforming RAT is optimal only for a small coverage area. When the UE moves, the serving beam can deteriorate fast due to the higher frequency, for which reason mobility is more challenging in the next-generation RATs. General approaches for centralized and distributed mobility management are discussed by F. Giust in doctoral dissertation “Distributed Mobility Management for a Flat Architecture in 5G Mobile Networks: Solutions, Analysis and Experimental Validation”, Universidad Carlos III de Madrid, 2015.